I used to use startup manager, as pointed out by Chad--24216, but alas, it's no longer being maintained, nor is it in the repositories.

It has been superseded by "grub-customizer", which while up to date, is also not in the standard repositories. To to add and install it simply do the following at the terminal:

sudo add-apt-repository ppa:danielrichter2007/grub-customizer

Press enter to confirm adding the PPA

sudo apt-get update
sudo apt-get install grub-customizer

Press Y to confirm

You can now launch grub-customizer in the usual ways. If you ignore the complete list you'll see when it first launches, and just press "Preferences" button on the toolbar you'll get a nice summary dialog where you can change the timeout value and default menu item, as shown below:

You need to edit /etc/default/grub file by opening a terminal and running this command: sudo gedit /etc/default/grub

You need to set the value of GRUB_TIMEOUT to how long (in seconds) you want the grub menu to appear. GRUB_TIMEOUT is actually the number of seconds before the default entry is automatically booted. If you want to see the menu for 10 seconds which is the default, then do:

Web-E's answer is really the best way to do it. However, if you want a GUI way of modifying GRUB that doesn't involve editing a file, you can download an install "startupmanager" from USC or Synaptic. Or with Universe repo enabled, you can open Terminal and install by typing: